PFS: rewrite, restyle

PFS: rewrite, restyle

- remove the buffer pool, inode bitmap, and inode hash table, and
  simplify the code accordingly;
- use theoretically slightly more optimal buffer management;
- put the entire source in one file, instead of having many files
  with one or two functions each;
- convert the code to KNF style.

Change-Id: Ib8f6f0bd99fbc6eb9098fba718e71b8e560783d9